U.S. patent number 5,552,027 [Application Number 08/503,563] was granted by the patent office on 1996-09-03 for working electrode for electrochemical enzymatic sensor systems.
This patent grant is currently assigned to Siemens Aktiengesellschaft. Invention is credited to Siegfried Birkle, Johann Kammermaier, deceased, Rolf Schulte.
United States Patent |
5,552,027 |
Birkle , et al. |
September 3, 1996 |
Working electrode for electrochemical enzymatic sensor systems
Abstract
A working electrode or an electrochemical-enzymatic sensor
system has a metallic base body which is provided with a thin layer
of amorphous hydrogenated carbon (a-C:H).
Inventors: |
Birkle; Siegfried (Hoechstadt,
DE), Kammermaier, deceased; Johann (late of
Unterhaching, DE), Schulte; Rolf (Erlangen,
DE) |
Assignee: |
Siemens Aktiengesellschaft
(Munchen, DE)
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Family
ID: |
6498332 |
Appl.
No.: |
08/503,563 |
Filed: |
July 18, 1995 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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287025 |
Aug 18, 1994 |
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Foreign Application Priority Data
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Sep 22, 1993 [DE] |
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43 32 251.4 |
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Current U.S.
Class: |
204/403.14;
148/240; 148/281; 148/DIG.1; 204/192.1; 204/192.12; 204/290.05;
204/290.08; 204/290.11; 204/400; 427/249.17; 427/327; 427/330;
427/450; 427/528; 427/569; 438/1; 438/49 |
Current CPC
Class: |
C12Q
1/001 (20130101) |
Current International
Class: |
C12Q
1/00 (20060101); G01N 027/26 (); C25B 011/00 () |
Field of
Search: |
;437/101,102,103,225,228-233
;427/450,528,569,249,255,255.1,255.2,327,330,419.1,419.2
;148/DIG.1,240,281 ;204/29F,403,400,192.1,192.12,192.16 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Koeberle, H. et al., "A Microstructural Investigation of Au-a-C:H
Films", Surface and Coatings Technology, vol. 39/40 (1989), pp.
275-284. No month available. .
Harnack, J. et al., "Target Effects During the Deposition of
Ti-a-C:H Films", Surface and Coatings Technology, vol. 39/40, pp.
285-292. No month/yr. available. .
IDR--Industrie Diamanten Rundschau, Bd. 18 (1984), p. 249-253. No
month available..
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Primary Examiner: Bell; Bruce F.
Attorney, Agent or Firm: Kenyon & Kenyon
Parent Case Text
This application is a continuation of application Ser. No.
08/287,025 filed Aug. 8, 1994 now abandoned.
Claims
What is claimed is:
1. A working electrode for an electrochemical-enzymatic sensor
system, comprising a base body of titanium, zirconium, molybdenum,
tungsten or electrically conductive silicon, the base body having a
thin layer consisting essentially of amorphous hydrogenated carbon
(a-C:H).
2. The working electrode according to claim 1 wherein the a-C:H
layer has a thickness of <5 .mu.m.
3. The working electrode according to claim 2 wherein the metallic
base body consists of titanium.
4. The working electrode according to claim 3 wherein the a-C:H
layer contains metal clusters.
5. The working electrode according to claim 2 wherein the a-C:H
layer contains metal clusters.
6. The working electrode according to claim 1 wherein the metallic
base body consists of titanium.
7. The working electrode according to claim 6 wherein the a-C:H
layer contains metal clusters.
8. The working electrode according to claim 1 wherein the a-C:H
layer contains metal clusters.
9. The working electrode according to claim 8 wherein the metal
clusters are clusters of gold or titanium.
10. A method of producing a working electrode for an
electrochemical-enzymatic sensor system, comprising the step of
depositing a thin layer consisting essentially of amorphous
hydrogenated carbon (a-C:H) on a base body of titanium, zirconium,
molybdenum, tungsten or electrically conductive silicon, by
low-pressure, high-frequency plasma deposition of a gaseous
hydrocarbon.
11. The method according to claim 10 wherein the plasma deposition
takes place at a pressure of 1 to 100 Pa.
12. The method according to claim 11 wherein the high-frequency
power density in the plasma is 0.1 to 10 W.cm.sup.-3 .
13. The method according to claim 11 wherein the plasma deposition
takes place with a DC self-bias voltage of up to 1100V.
14. The method according to claim 10 wherein the high-frequency
power density in the plasma is 0.1 to 10 W.cm.sup.-3.
15. The method according to claim 14 wherein the plasma deposition
takes place with a DC self-bias voltage of up to 1100V.
16. The method according to claim 10 wherein the plasma deposition
takes place with a DC self-bias voltage of up to 1100V.
17. The method according to claim 10 wherein the hydrocarbon used
is methane.
18. The method according to claim 10 wherein metal clusters are
incorporated in the a-C:H layer.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a working electrode for
electrochemical-enzymatic sensor systems and to a method of
producing such a working electrode.
2. Description of Related Art
For the medical detection of metabolic processes in body fluids,
for instance, glucose reaction, sensor systems are used in which
changes in electrochemical potential which are produced by
enzymatic reactions serve as characteristic value. The following
requirements are made on the working electrodes of such sensor
systems:
electrical conductivity >10.sup.-2 .OMEGA..sup.-1 cm.sup.-1
variability of shape for special electrode geometries
chemically inert character, particularly with regard to the
electrochemically produced reaction products
compatibility of the electrode material with the human body
smooth surfaces without roughnesses or grain boundaries for
avoiding the danger of a thrombosis upon the implantation.
From EP-OS 0 470 290 an electrochemical-enzymatic sensor for the
determining of substances in body fluids, particularly glucose, is
known. This sensor has the following characteristics:
a sensor electrode of electrocatalytically inactive carbon
a counter electrode
a reference electrode
an enzyme-containing layer present in front of the sensor
electrode, and
a membrane of biocompatible, hydrophilic oxygen-pervious material
which covers the enzyme layer off from the body fluid and holds the
enzyme back.
In this sensor, the material for the sensor electrode, which is
also known as working electrode or measurement electrode, consists
of vitreous carbon, pyrographite, sputtered carbon, sputtered
graphite or amorphous hydrogenated carbon. Vitreous carbon is
preferred, namely in the form of a smooth vitreous-carbon
electrode.
Vitreous carbon is generally produced by pyrolysis of polyfurfuryl
alcohol. This material, as carbon modification--is sufficiently
inert chemically and, on basis of its amorphous structure, it has a
smooth surface. It is suitable for implantation. Vitreous carbon
also satisfies the requirements with regard to electric
conductivity. However, one disadvantage is that vitreous carbon is
difficult to contact. Furthermore, vitreous carbon requires costly
processing since cracks can easily occur in thin layers upon the
high-temperature pyrolysis involved in its production.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a working
electrode for electrochemical-enzymatic sensor systems which fully
satisfies the requirements made on such an electrode.
This is achieved, in accordance with the invention, by providing a
working electrode having a metallic base body which is provided
with a thin layer of amorphous hydrogenated carbon (a-C:H).
DETAILED DESCRIPTION OF THE INVENTION
Amorphous hydrogenated carbon is a carbon modification having
diamond-like mechanical and chemical properties, which forms
pinhole-free, smooth surfaces (see, for instance: "IDR-Industrie
Diamanten Rundschau" Vol. 18 (1984) , pages 249 to 253). Surfaces
coated with a-C:H are tissue compatible in the same way as vitreous
carbon and are inert with respect to body fluids, such as blood.
One basic difference from diamond is that the electrical resistance
can be controlled intrinsically within a wide range by the
conditions of deposition, conductivities which are comparable even
to graphite are obtainable (up to 10.sup.-1 .OMEGA..sup.-1
cm.sup.-1 ). The amorphous structure of a-C:H results from the
presence of sp.sup.3 and sp.sup.2 bonds of the C atoms and it is
stabilized by chemically bound H atoms (maximum proportion: about
60 vol. %) at temperatures up to about 500.degree. C.
The a-C:H layer preferably has a thickness of <5 .mu.m. The
metallic base body consists preferably of titanium; in addition,
however, metals such as zirconium, molybdenum and tungsten are also
suitable, as well as highly doped, electrically conductive
silicon.
For the production of the working electrode of the invention and
for the deposition of the thin layer of a-C:H on the metallic base
body, a low-pressure plasma deposition with high frequency
excitation is used; the operating frequency is for instance 13.56
MHz. Hydrocarbons, particularly CH.sub.4, C.sub.2 H.sub.4 and
C.sub.6 H.sub.6, are used as process gas, preferably at an
operating pressure of between 1 and 100 Pa. The HF power density in
the plasma is preferably between 0.1 and 10 W.cm.sup.-3. With
unequal electrode surfaces (ratio between about 0.1 and 0.5) a DC
self-bias voltage of up to 1100V is established, the smaller
electrode being the cathode. Due to this DC self-bias voltage, the
a-C:H formation takes place predominantly as an ion deposition
process with a very high energy of the C.sub.x H.sub.y particles
impinging on the substrate. In this way, there is obtained both a
high proportion of diamond-like sp.sup.3 bonds of the C atoms in
the a-C:H layer and also--due to a carbide formation in the
boundary layer--a high adherence strength of the a-C:H layer to the
metallic base body. In contrast to this, this is not true upon the
CVD deposition of pyrocarbon or upon the pyrolytic deposition of
vitreous carbon.
The shape of the electrodes (in the deposition reactor) is adapted
to the shape of the metallic base body to be coated. Thus, for
instance, in the case of flat substrate surfaces, electrodes of
different sizes having flat, parallel surfaces are used, while in
the case of curved base bodies correspondingly shaped, for instance
spherically shaped, pairs of electrodes are employed in order to
obtain a sufficient DC self-bias effect.
One advantageous embodiment of the invention is in including metal
clusters of, for instance, gold or titanium in the a-C:H layer
during the plasma deposition from a target present in the
deposition reactor in order to improve the electric conductivity
(see in this connection also; "Surface and Coatings Technology,"
Vol. 39/40 (1989), pages 275 to 284 and 285 to 292). In this case,
the a-C:H deposition is carried out under changed conditions, i.e.
the substrate is positioned on the anode. The cathode bears the
target from which the metal atoms are split off by impinging
C.sub.x H.sub.y ions or Ar ions.
The invention will be further explained on the basis of
embodiments.
EXAMPLE 1
An a-C:H layer of a thickness of about 1.8 .mu.m is deposited on a
flat sheet of titanium of a thickness of 0.25 mm which has been
cleaned with a solvent and subjected to a preliminary plasma
treatment with argon in accordance with the conditions described
below. Methane is used as process gas; the operating pressure is 20
Pa. With an electrode-surface ratio of about 0.16 between the
electrode bearing the substrate and the cup-shaped counter
electrode--with an operating frequency of 13.56 MHz and an HF power
density of 0.3 W.cm.sup.-3 --a DC self-bias voltage of about 850V
is established in the plasma, the smaller electrode (surface: 44.2
cm.sup.2) being the cathode. The a-C:H layer obtained has a
specific electric resistivity of about 10.sup.4 .OMEGA..cm. The
titanium electrode which has been coated in this manner exhibits a
very small double-layer capacitance of 4.4 .mu.F.cm.sup.-2 in
electrochemical measurements, which indicates a very smooth and
dense a-C:H coating.
EXAMPLE 2
In the same way as in Example 1, with an operating pressure of 3.5
Pa, an HF power density in the plasma of 6.5 W.cm.sup.-3, a DC
self-bias voltage of about 1100V, and using flat electrodes with a
surface ratio of about 0.27, other conditions being the same, a-C:H
layers of a thickness of 0.4 .mu.m having an electrical resistance
of only 7 .OMEGA..cm are obtained on base bodies of titanium.
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